The present invention is directed to liquid ink compositions. More specifically, the present invention is directed to aqueous based ink compositions particularly suitable for use in ink jet printing processes. One embodiment of the present invention is directed to an ink composition which comprises an aqueous liquid vehicle, a water-soluble dye, and particles of a block copolymer of the formula ABA, wherein A represents a hydrophilic segment and B represents a hydrophobic segment, said ABA particles having an average diameter of about 300 Angstroms or less. Optionally, silica is precipitated within the micelles.
Ink jet printing systems generally are of two types: continuous stream and drop-on-demand. In continuous stream ink jet systems, ink is emitted in a continuous stream under pressure through at least one orifice or nozzle. The stream is perturbed, causing it to break up into droplets at a fixed distance from the orifice. At the break-up point, the droplets are charged in accordance with digital data signals and passed through an electrostatic field which adjusts the trajectory of each droplet in order to direct it to a gutter for recirculation or a specific location on a recording medium. In drop-on-demand systems, a droplet is expelled from an orifice directly to a position on a recording medium in accordance with digital data signals. A droplet is not formed or expelled unless it is to be placed on the recording medium.
Since drop-on-demand systems require no ink recovery, charging, or deflection, the system is much simpler than the continuous stream type. There are two types of drop-on-demand ink jet systems. One type of drop-on-demand system has as its major components an ink filled channel or passageway having a nozzle on one end and a piezoelectric transducer near the other end to produce pressure pulses. The relatively large size of the transducer prevents close spacing of the nozzles, and physical limitations of the transducer result in low ink drop velocity. Low drop velocity seriously diminishes tolerances for drop velocity variation and directionally, thus impacting the system's ability to produce high quality copies. Drop-on-demand systems which use piezoelectric devices to expel the droplets also suffer the disadvantage of a slow printing speed.
The other type of drop-on-demand system is known as thermal ink jet, or bubble jet, and produces high velocity droplets and allows very close spacing of nozzles. The major components of this type of drop-on-demand system are an ink filled channel having a nozzle on one end and a heat generating resistor near the nozzle. Printing signals representing digital information originate an electric current pulse in a resistive layer within each ink passageway near the orifice or nozzle, causing the ink in the immediate vicinity to evaporate almost instantaneously and create a bubble. The ink at the orifice is forced out as a propelled droplet as the bubble expands. When the hydrodynamic motion of the ink stops, the process is ready to start all over again. With the introduction of a droplet ejection system based upon thermally generated bubbles, commonly referred to as the "bubble jet" system, the drop-on-demand ink jet printers provide simpler, lower cost devices than their continuous stream counterparts, and yet have substantially the same high speed printing capability.
The operating sequence of the bubble jet system begins with a current pulse through the resistive layer in the ink filled channel, the resistive layer being in close proximity to the orifice or nozzle for that channel. Heat is transferred from the resistor to the ink. The ink becomes superheated far above its normal boiling point, and for water based ink, finally reaches the critical temperature for bubble formation or nucleation of around 280.degree. C. Once nucleated, the bubble or water vapor thermally isolates the ink from the heater and no further heat can be applied to the ink. This bubble expands until all the heat stored in the ink in excess of the normal boiling point diffuses away or is used to convert liquid to vapor, which removes heat due to heat of vaporization. The expansion of the bubble forces a droplet of ink out of the nozzle, and once the excess heat is removed, the bubble collapses on the resistor. At this point, the resistor is no longer being heated because the current pulse has passed and, concurrently with the bubble collapse, the droplet is propelled at a high rate of speed in a direction towards a recording medium. The resistive layer encounters a severe cavitational force by the collapse of the bubble, which tends to erode it. Subsequently, the ink channel refills by capillary action. This entire bubble formation and collapse sequence occurs in about 10 microseconds. The channel can be refired after 100 to 500 microseconds minimum dwell time to enable the channel to be refilled and to enable the dynamic refilling factors to become somewhat dampened. Thermal ink jet processes are well known and are described in, for example, U.S. Pat. Nos. 4,601,777, 4,251,824, 4,410,899, 4,412,224, and 4,532,530, the disclosures of each of which are totally incorporated herein by reference.
Known ink jet inks generally comprise a water soluble dye which is soluble in an ink vehicle such as water or a mixture comprising water and a water soluble or water miscible organic solvent. Heterophase ink jet inks are also known.
U.S. Pat. No. 4,877,451 (Winnik et al.), the disclosure of which is totally incorporated herein by reference, discloses ink jet ink compositions comprising water, a solvent, and a plurality of colored particles comprising hydrophilic porous silica particles to the surfaces of which dyes are covalently bonded through silane coupling agents. In addition, copending application U.S. Ser. No. 07/369,003, the disclosure of which is totally incorporated herein by reference, discloses ink jet inks and liquid developers containing colored particles comprising hydrophilic porous silica particles to the surfaces of which dyes are covalently bonded through silane coupling agents.
U.S. Pat. No. 4,228,438 (Vazirani) discloses a jet electrostatic printing process which uses an ink formulation suitable for use on dark colored surfaces. The ink comprises a prepolymer, pigments, and additives such as coupling agents, surface modifiers, and conductivity enhancing materials. The prepolymer may comprise aliphatic and aromatic polymers. The pigment particle size is less than 5 microns, which results in a uniform droplet weight. The additives may include silane coupling agents.
U.S. Pat. No. 4,597,794 (Ohta et al.) discloses an ink jet recording process which comprises forming droplets of an ink and recording on an image receiving material by using the droplets. The ink is prepared by dispersing fine particles of pigment into an aqueous dispersion medium containing a polymer having both a hydrophilic and a hydrophobic construction portion. The particle diameter of the pigment is in microns and is related to the molecular weight of the pigment.
U.S. Pat. No. 4,680,332 (Hair et al.) discloses a heterophase ink composition comprising a water insoluble polymer dispersed in a liquid medium, the polymer containing therein an oil soluble dye, and a nonionic stabilizer permanently attached thereto. The particles have diameters of 0.5 micron or less to prevent nozzle clogging.
U.S. Pat. No. 4,783,220 (Gamble et al.) discloses ink compositions consisting of small unilamellar or multilamellar vesciles formed from surfactants of anionic, cationic, zwitterionic, and nonionic molecules having an oil soluble dye, inclusive of a lipid-soluble dye associated therewith. The dye to surfactant ratio is preferably from about 1:1 to about 1:10 by weight. The resulting ink is from about 100 to about 400 nanometers in diameter. The lipid soluble dye molecules have a hydrophobic and a hydrophilic region. The compositions are useful in traditional printing techniques such as flexography and rotogravure and in electronic printing systems such as with an ink jet printer.
U.S. Pat. No. 4,836,852 (Knirsch et al.) discloses an ink formed by a solution of a direct dye in a mixture of water and glycol wetting agents, to which a pigment which is finely ground to particles of dimensions of not more than 1000 Angstroms is added in dispersion in a concentration of between 0.1 and 2 percent. The pigment particles serve to anchor the gaseous nuclei of gases which are dissolved in the ink for the purpose of stabilizing the boiling point of the ink. The ink is particularly suited to an ink jet printer of the type in which expulsion of the droplets is produced by causing instantaneous vaporization of a portion of ink in a nozzle.
Copending Application U.S. Ser. No. 544,564, entitled "Inks Containing Colored Block Copolymer Micelles", with the named inventors Francoise Winnik, Peter Hofstra, and Paul Gerroir, the disclosure of which is totally incorporated herein by reference, discloses an ink composition which comprises an aqueous liquid vehicle and colored particles of an average diameter of 50 nanometers or less which comprise micelles of block copolymers of the formula ABA, wherein A represents a hydrophilic segment and B represents a hydrophobic segment, and wherein dye molecules are covalently attached to the micelles. Optionally, silica is precipitated within the micelles. Also disclosed is an ink preparation process which comprises, in the order stated (1) adding to water a block copolymer of the formula ABA, wherein A represents a hydrophilic segment and B represents a hydrophobic segment, thereby forming a dispersion of micelles of the block copolymer; (2) adding a water-soluble base to the dispersion, thereby bringing the pH of the dispersion to at least 8; (3) adding to the dispersion a solution comprising water and a reactive dye capable of reacting with the block copolymer, thereby forming colored polymeric micelles; and (4) admixing the colored micelles with an aqueous liquid vehicle to form an ink composition. Optionally, silica is precipitated in the micelles by addition of a tetraalkoxysilane to the micelles prior to addition of the water-soluble base.
Although known compositions and processes are suitable for their intended purposes, a need remains for improved ink compositions suitable for use in ink jet printing processes. In addition, a need remains for ink compositions that enable increased drop volume when used in ink jet printing processes. Further, there is a need for ink composition that exhibit improved waterfastness. There is also a need for ink compositions that exhibit improved print quality. Additionally, there is a need for ink compositions that enable the above advantages and also exhibit good drying times and optical densities. In addition, there is a need for ink compositions that enable control of the ink drop volume in an ink jet printer without the need for printhead modifications.